Oracle Blog

The Directory Services

Wednesday Dec 16, 2009

Rationale

Why do caching in DPS ?

The Directory Server back-ends are not able to "absorb" as many updates when they're stressed with a large proportion of searches. After all there's already caching on the back-end Directory Server itself. It helps a lot performance, since reads return to the client faster, it relieves some of the stress and frees up resources to take care of the writes that lock resources for longer atomic stretches of time. But as long as searches hit the back-end, even with cache, there's some weight lifting to be done: open the connection, parse the request, put the request in the work queue, lookup entries in the cache, return the entry, close the connection...

That's why caching right in DPS started to look appealing to me.

Why Coherence ?

Well, as much as one may think it is because Sun is about to be gobbled up by Oracle that I made this choice but the answer is no. Coherence is simply a compelling choice, these guys seem to have made all the technical choices I would have ... and then some. For one, you download the bits, then just start it and it works. It may sound like a marketing pitch but see for yourself. Excellent 0 to 60 in my book. Once you have it working, you get acquainted with it, the protocol is dead simple, the API is clean, robust and pretty lean. After that, you check the docs out (that's right, I read the docs after the fact) and start to realize how powerful a platform it is, how expandable it is, how far you can push deployments to accommodate growing performance or reliability needs.

Bird's Eye View

The integration with Coherence is done by way of DPS (7+) plug-in that will -asynchronously- populate a Coherence cache with entries being returned by your regular traffic. When requests come in, a lookup is done to check if the entry is present in the cache and returned immediately if it is the case, otherwise the request is routed to the back-end as usual.

Note that I'm not making any claims on the performance aspect from the client's perspective of this caching approach because our Directory Server back-end is already pretty darn fast. This for sure relieves it from a bit of "frequent" traffic and will certainly benefit the overall performance of the topology. The relief will most certainly result in improved write response times but nothing speaks to the performance of the Coherence cache lookup. I just haven't collected enough data so far.